Earlier today I was running some errands, and I was on a freeway overpass trailing behind a service truck carrying a port-a-potty. Fortunately there was another car in front of me to create a buffer between myself and the truck. That car was getting dangerously close to that port-a-potty. The truck hit a bump and I saw the port-a-potty shake (it only seemed to be secured by a single strap), and I thought it was going to fall off and hit that car.
Fortunately, that didn't happen, and we all went our separate ways.
But I realized that it's always a good idea to leave some distance between yourself and the port-a-potty.